Active hardmask for lithographic patterning

ABSTRACT

In an implementation, energy reaching the lower surface of a photoresist may be redirected back into the photoresist material. This may be done by, for example, reflecting and/or fluorescing the energy from a hardmask provided on the wafer surface back into the photoresist.

BACKGROUND

Photolithographic techniques may be used in the fabrication ofsemiconductor devices to transfer a pattern (e.g., a circuitry pattern)to a wafer using light or other wavelengths of electromagneticradiation. The “light” may pass through, or be reflected off of, a maskwhich defines the pattern. Light from the mask projects an image of thepattern onto the wafer. The wafer is coated with a layer of aphotosensitive material, referred to as a “photoresist” or “resist,”which undergoes a chemical reaction when exposed to light. Afterexposure, the resist is baked and developed, leaving regions of thewafer surface covered resist and complementary regions exposed.

The resolution and efficiency of photolithographic systems may beaffected by the amount of light coupled into the photoresist. Theoptical absorbance of photoresist materials used in lithography hasincreased with decreasing wavelength, especially in the ExtremeUltra-Violet, or EUV (˜13.5 nm). As a result less light reaches theunderlying substrate (˜50% in the EUV). A decreasing index of refractionmismatch between the photoresist and the underlying substrate has alsoreduced reflections from the substrate back into the photoresist. Thenet effect is wasted photons.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a multilayer coating applied onto a substrate.

FIG. 1B shows a list of some possible material combinations and theirreflectivities.

FIG. 2 a shows the calculated reflectivity spectrum of representative10-period multilayers of Mo/Si and Ru/Si, at normal incidence forunpolarized light.

FIG. 2 b shows a plot of the reflectivity of a multilayer reflector forvarious numbers of layers.

FIG. 3 a shows a plot showing the impact of a multilayer reflectivehardmask on photoresist profile.

FIG. 3 b shows a plot showing the impact of a multilayer reflectivehardmask on photoresist profile and on dose to size.

FIG. 4 shows a plot showing the impact of diffusion length on the effectof standing waves in the EUV spectral range.

FIG. 5 shows a hardmask reflecting energy back into photoresistmaterial.

FIG. 6 shows a hardmask fluorescing energy back into photoresistmaterial.

FIG. 7 shows a flowchart outlining a method of redirecting EUV energyinto a resist.

DETAILED DESCRIPTION

Lithography may be performed using various frequencies ofelectromagnetic radiation. Radiation transmitted from a patterned maskmay be coupled into a photoresist material on a wafer. The exposedportions of the photoresist undergo a chemical reaction, e.g., byphotosolubization in positive resists or polymerization in negativeresists. The amount of energy (photons) coupled to the photoresist mayaffect system throughput, pattern transfer, and resolution.

Because reflections occur at interfaces when there is an index ofrefraction mismatch between the materials on each side of the interface,in Deep UV (DUV) lithography reflections from the top and bottom resistinterfaces may be so strong that prominent standing waves may be createdin the resist. Photons may be absorbed in a first pass as the lightenters the photoresist and in a second pass as photons are reflectedfrom the substrate surface. To improve resolution and critical dimension(CD) control, standing waves may be minimized in DUV lithography by theuse of anti-reflective hardmasks (bottom ARCS) to control thereflectivity at the interface.

At extreme UV (EUV) frequencies (e.g. 13.5 nm), however, standing wavesdo not pose a problem because there are very few reflections from theresist-substrate interface because the real part of the index ofrefraction of materials is close to 1. The lack of reflection means thatthe energy absorbed in a layer of photoresist in EUV lithography systemsoccurs only during a single pass of EUV photons through the resist.

The resist materials typically used in EUV lithography have a highabsorbance of energy. Consequently, much of the EUV energy is absorbedin the upper portions of the resist. Together with the lack of areflection in EUV, the high absorbance of the resist at EUV frequenciesresults in the lower portions of the resist receiving little of the EUVenergy. This, in turn, may cause problems in producing verticalsidewalls in the resist pattern; the top of the resist receives moreenergy than the bottom so, in a positive resist, the top of the channel(areas that will clear after the develop step) will be wider than thebottom. Conversely, in a negative resist, the top of the channel will benarrower than the bottom.

These undesirable results may be reduced by, for example, redirectingsome of the energy that reaches the lower surface of the photoresistback into the photoresist. In an implementation, the optical propertiesof the hardmask may be tuned to make it reflective. Thus, as illustratedin FIG. 5, instead of acting as a bottom anti-reflective coating (BARC)to minimize reflection, the hardmask 501 does just the opposite; itreflects EUV energy 503 back into the resist material 505. As a result,instead of receiving only one pass of EUV photons, the lower portions ofthe resist 505 are exposed to additional EUV photons 507 that wouldotherwise have simply passed through the bottom surface of the resist505. Because the lower portions of the resist 505 have absorbed moreradiation, the resulting channel sidewalls exhibit an improved profileand are closer to the desired vertical angle.

Although the hardmask 501 is shown as a single layer, it may also becomprised of multiple-layers, as discussed below. Furthermore, althoughthe reflected EUV photons 507 are shown in FIG. 5 as originating at theinterface between the resist 505 and the hardmask 501, it should beunderstood that some or all of the reflection may occur within thehardmask 501.

In addition, because light is being reflected back up into the resist,less overall light is required to expose the resist overall. As aresult, the run rate of the lithography tools may be increased.

As shown in FIG. 1A, a substrate 101 with a layer to be patterned iscoated with a reflective hard mask In this configuration, the reflectivehardmask is comprised of a multi-layer (ML) reflective coating 103. A MLcoating is necessary for EUV since no material reflects appreciably atEUV wavelengths. This is because the real part of the index ofrefraction for all materials at these frequencies is roughly equal toone. One example of a ML coating 103 is an appropriate Bragg reflector.At longer wavelengths, such as vacuum UV (VUV), e.g. 157 nm, where thereal part of the index of refraction varies for different materials,hardmask material of appropriate thickness made of a single layer or afew layers may be chosen.

As mentioned above, the ML structure 103 may employ a Bragg reflector,which is a filter that works on interference properties. In a multilayerBragg reflector, reflections from each layer add up coherently, inphase, such that even though there is little reflectivity from any oneinterface, the total reflectivity from many interfaces adds upconstructively to be substantial.

In an implementation, the multilayer reflector will produce constructiveinterference if each layer meets the quarter wavelength criterion, i.e.the thickness of the layer should conform to the following equation:L=λ/(4n), where L is the thickness of the layer, λ is the wavelength ofthe light in a vacuum, and n is the real portion of the index ofrefraction of the layer material.

As shown in FIG. 1A, two different materials, here designated A 105 andB 107, may be used to create the multilayer reflector 103. The substrate101 is repeatedly coated alternately with materials A and B. In oneimplementation, the ML coating 103 contains at least ten pairs of A/Blayers.

However, the reflective stack 103 may be created from a variety ofmaterial combinations that are compatible with standard semiconductorprocess manufacturing equipment. Standard physical vapor depositiontools may be configured for dual-chamber, in-situ deposition of themultilayers on several wafers simultaneously. In one aspect, depositionmay occur on as many as four wafers at once. Furthermore, certaincandidate materials for the reflective hardmask may be used as metalgates, which may provide an opportunity for tool synergy and capitalcost reduction. A non-exclusive list of some possible materialcombinations 151 and their reflectivities 153 is provided in FIG. 1B.

In an implementation, molybdenum/silicon (Mo/Si) may be used. In anotherimplementation, ruthenium/silicon (Ru/Si) may be used. Ru/Si has veryfavorable optical properties, but the process of depositing adefect-free, thin film of ruthenium onto silicon may be difficult. As aresult, Mo/Si is commonly used in creating reflective coatings.

FIG. 2 a shows the calculated reflectivity spectrum of representative10-period multilayers of Mo/Si 201 and Ru/Si 203, at normal incidencefor unpolarized light. As indicated in the figure, the reflectivity peakof the Mo/Si-stack was found to be 33% at 13.5 nm. The peak of the Ru/Siwas found to be 45% at 13.5 nm. As FIG. 2 b shows, increasing the numberof periods of the Ru/Si stack increases the calculated reflectivity 251even further. By sixteen periods, the reflectivity of a Ru/Si stack 253increases to 60%, although the addition of subsequent layers increasesthe reflectivity 251 more slowly.

Although these calculations indicate that Ru/Si is superior to Mo/Si,measured reflectivity of actual samples indicates the opposite,primarily due to greater interfacial diffusion and roughness in theRu/Si. The multilayer hardmask reflectivity obtained in a workableprocess flow, therefore, will be lower than the theoretical limitsreported here. The final choice in material may depend on many factors,for example, stability, throughput of the deposition tool, andcompatibility with gate materials. As technology improves upon these andother factors in deposition processes, the choice of materials used inthe multilayer may change.

Using a reflective hardmask allows the lower levels of the resist toabsorb more light, thus improving both the sidewall angle and the amountof light exposure needed for development (which is inversely related torun rate). FIGS. 3 a and 3 b, illustrating simulations with 120 nm of atypical EUV resist patterning 100 nm lines/spaces with NA of 0.1 donewith and without a Ru—Si reflective hardmask, demonstrate theseimprovements. As shown in FIG. 2 b, increasing the number of multilayersincreases the reflectivity of the hardmask; FIGS. 3 a and 3 b show theresulting improvements to resist profile 352 and dose to size 353 (ametric related to required exposure time). For example, fifteen periodsof Ru—Si in the reflective multilayer was calculated to improve sidewallangle by 5 degrees and increase run rate by 30%. Thus, although theremay be an increased complexity and apparent financial cost involved inadding a new process step, the improved sidewall profile and increasedrun rate may provide an actual cost reduction in addition to higherquality wafers. In one implementation, this improvement in run rate (orincrease in photo speed) may be accomplished without significant changesto the resist being used.

Standing waves do not pose a significant problem in conventional EUVlithography because there is virtually no light that reflects off thebottom of the resist. The use of a reflective hardmask, though, mayincrease the standing waves in the resist. These EUV standing waves havea small periodicity (approximately λ/4n, where n=index of refraction ofthe resist (in one aspect, approximately 1) and λ=wavelength (in oneaspect, 13.4 nm)). Thus, a small amount of diffusion duringpost-exposure bake (in one aspect, approximately 4 nm) may be sufficientto smooth out the effects of the standing waves.

FIG. 4 illustrates the effects of this process. The sidewall profile 401may be adequately smoothed by choosing an appropriate diffusion length.

In an alternative implementation, the hardmask may be fluorescent tosend energy back into the resist. Thus, as illustrated in FIG. 6, someenergy 603 that passes through the resist 605 is fluoresced 607 by thehardmask 601 back into the lower surface of the resist 605. This mayimprove either or both the photo speed (amount of light/radiationexposure) and the resulting sidewall profiles. For example, the materialSTI-F10G, available from Star Tech Instruments in Danbury, Conn., hasbeen demonstrated to fluoresce when excited by wavelengths from softx-rays to DUV with submicron resolution. For longer DUV wavelengths,teraphenyl butadiene has shown much higher fluorescent efficiencies(peaking over 90% into 4 pi near 157 nm). Regardless of whicheverfluorescent material is used, in this implementation the hardmaskfluoresces into a spectral region where the resist is sensitive. Forexample, EUV resist are very sensitive in the deep UV range. Thus, inone implementation, the fluorescent hardmask absorbs EUV light (e.g.13.5 nm) and fluoresces or re-radiates it in the deep UV range. Inanother implementation, the hardmask both fluoresces and reflects energyback into the resist.

Although the hardmask 601 is shown as a single layer, it may also becomprised of multiple-layers. Furthermore, although FIG. 6 shows thefluoresced energy 607 as originating within the hardmask 601, it shouldbe understood that some or all of the fluorescence may occur at theinterface between the hardmask 601 and the resist 605.

FIG. 7 is a flowchart describing a method 700 for redirecting EUV energyinto a photoresist. A hardmask (reflective and/or fluorescent to EUVwavelengths) may be applied to a surface of a wafer (block 705). Aphotoresist layer may then be applied over the hardmask (block 710). Theresist may be exposed with EUV radiation from a mask defining a pattern(block 715) The hardmask may redirect 33% or more of a desiredwavelength of EUV radiation (e.g., 13.5 nm) reaching the hardmask, ifreflective, into the photoresist and/or DUV radiation if fluorescing(block 720).

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. Accordingly, other embodimentsmay be within the scope of the following claims.

1. A method comprising: directing extreme ultraviolet electromagneticenergy from an energy source toward a photoresist, the photoresisthaving a first surface closer to the energy source than a secondsurface; and redirecting a portion of the electromagnetic energyreaching the second surface back into the photoresist, wherein theredirecting comprises fluorescing the portion of the electromagneticenergy.
 2. The method of claim 1, wherein the redirecting comprises bothfluorescing and reflecting the portion of the electromagnetic energy. 3.A method comprising: directing extreme ultraviolet electromagneticenergy from an energy source toward a photoresist, the photoresisthaving a first surface closer to the energy source than a secondsurface; redirecting a portion of the electromagnetic energy reachingthe second surface back into the photoresist, wherein the redirectingcomprises reflecting the portion of the electromagnetic energy, andwherein the reflecting comprises reflecting with a Bragg reflector. 4.The method of claim 3, wherein the reflecting comprises reflecting witha reflector comprising multiple layers and wherein the multiple layerscomprise ruthenium and silicon.
 5. A method comprising: directingextreme ultraviolet electromagnetic energy from an energy source towarda photoresist, the photoresist having a first surface closer to theenergy source than a second surface; redirecting a portion of theelectromagnetic energy reaching the second surface back into thephotoresist, wherein said redirecting comprises redirecting at least 33%of the electromagnetic energy reaching the second surface.
 6. A methodfor transferring a pattern to a semiconductor substrate, the methodcomprising: applying a reflective hardmask above a surface of thesemiconductor substrate; applying a photoresist to a surface of thereflective material; directing extreme ultraviolet energy toward thephotoresist; and reflecting at least a portion of the energy back intothe photoresist; wherein the applying the reflective hardmask comprisesapplying a Bragg reflector.
 7. The method of claim 6, wherein theapplying the reflective hardmask comprises applying a plurality oflayers, and wherein the plurality of layers comprise ruthenium andsilicon.
 8. The method of claim 6, further comprising fluorescingradiation into the photoresist.
 9. The method of claim 6, wherein thereflecting at least a portion of the energy back into the photoresistcomprises reflecting back into the photoresist at least 33% of theextreme ultraviolet energy reaching a lower surface of the photoresist.